Problems with using VirtualBox guest properties for reading NFS Guest IP

So after #2626 and the newest comments on #1437 about Vagrant 1.4, I think it's time to re-think the strategy we switched to for getting the VirtualBox Guest IP for NFS.

In #2560 we switched from pulling (and requiring) a static guest IP from the config to fetching the actual assigned IP from a vbox `guestproperty`.

This allowed DHCP interfaces to work, but it introduced a dependency on vbox guest additions loading and populating the value. So guests that either take too long or don't have guest additions for their platform ended up SOL even with static IPs.

As I type this, I'm realizing the best option is probably to bring back the config-interpreting strategy for static IPs, so we no longer break people who were working fine with `1.3.x`. We can leave the `guestproperty` strategy, but limit it to Vagrantfiles that only have DHCP interfaces.

Comments are welcome. I'll track any progress I make here.

/cc @viq @mitchellh